1. Field of the Invention
The present invention relates to a power source circuit having a function of supplying power via a communication line.
2. Description of the Related Art
Recently, there is known a method of feeding power from a power source to an apparatus via an Ethernet® cable. This method complies with the IEEE802.3af standard and is called Power over Ethernet® (to be referred to as PoE). In the IEEE802.3af standard, when a predetermined current or more is consumed, the PoE feed device activates overload protection to stop feeding from the PoE power source. The maximum limit current value of overload protection is 400 mA. The minimum limit current value is obtained by dividing a maximum power consumption of 15.4 W by the PoE feed voltage. The PoE feed voltage ranges from DC 36 V to DC 57 V. That is, the overcurrent limit value is 270 mA to 400 mA when the PoE feed voltage is DC 57 V, 320 mA to 400 mA when it is DC 48 V, and 400 mA when it is DC 37 V.
Of the apparatuses designed to operate by the PoE power source, some are designed to be operated by even a general-purpose power source such as an AC adapter other than the PoE power source due to the convenience of installation. There is known an apparatus which, when both the PoE power source and general-purpose power source are connected, operates by giving priority to feeding from either power source (for example, Japanese Patent Laid-Open No. 2007-36832). The following problem arises in conventional specifications which give priority to the PoE power source.
Upon switching from a state in which the apparatus operates by the AC adapter (for example, DC 12 V) to a state in which it operates by the PoE power source (for example, DC 48 V), a subsequent switching power source unit instantaneously tries to draw a current consumed at DC 12 V directly from DC 48 V. Since the voltage switches from DC 12 V to DC 48 V, a further inrush current is generated for a capacitive component at the input stage of the switching power source unit. As a result, more than a specified current is drawn from the PoE control device. Overcurrent protection of the PoE feed device acts, stopping supply from the PoE power source.
For this reason, a power source circuit with a large amount of load power that gives priority to the PoE power source generally permits the power source to be stopped upon switching to the PoE power source. This problem occurs not only in the PoE power source but also in a power source which stops feeding upon consumption of a predetermined current or more. Another conventional method uses an input voltage from the general-purpose power source after boosting it to a high voltage. The difference between the PoE power source and the general-purpose power source is reduced to prevent an overcurrent from flowing even upon switching to the PoE power source.
However, this conventional method needs to add a booster circuit. This results in high component cost, large substrate size, low power source efficiency, and large amounts of heat being generated.